llvm/llvm-project
Auf GitHub ansehenTake advantage of `assume`s on `x * x` (with `nuw`)
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#122.412 geöffnet am 10. Jan. 2025
good first issuellvm:optimizationsmissed-optimization
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Beschreibung
Rust stabilized its isqrt method today, so I was looking at its codegen.
The output for
let root = u16::isqrt(num);
currently has a manual
assume(root <= 255)
I was thinking that it'd be nice to give LLVM a bit more specific information, like
assume(root * root <= num)
since that's (a partial) definition of what isqrt does, and strictly more specific.
Sadly, that assume today doesn't seem to be worth bothering adding, since opt seems like it doesn't take advantage of it even in a "simple" situation that doesn't need to know anything about num: https://llvm.godbolt.org/z/zoq4T5fKr
But Alive2 confirms that it would be allowed to https://alive2.llvm.org/ce/z/ko8HYR
define i1 @src(i16 noundef %num, i16 noundef %s) noundef zeroext {
start:
%_4 = mul nuw i16 noundef %s, noundef %s
%cond = icmp ule i16 %_4, noundef %num
assume i1 %cond
%_0 = icmp ult i16 noundef %s, 256
ret i1 %_0
}
=>
define i1 @tgt(i16 noundef %num, i16 noundef %s) noundef zeroext {
start:
ret i1 1
}
Transformation seems to be correct!
Original Rust code I used to get the LLVM IR: https://rust.godbolt.org/z/vM8qj4xjf
pub unsafe fn isqrt_facts(num: u16, s: u16) -> bool {
std::hint::assert_unchecked(u16::unchecked_mul(s, s) <= num);
s <= 255
}